The present invention relates to electroless plating of polysulfone and other polymers and to improving coverage and bond strength between an electrolessly deposited metal coating and the polymer substrate.
A considerable demand exists for metal plated non-conductive articles, particularly plastic articles. In the finished product the desirable characteristics of the plastic and metal are combined to offer thereby the technical and aesthetic advantages of each. For instance, superior mechanical properties of polysulfone resins may be aesthetically enhanced by a metal coating. Although the polysulfones, like most polymers, are electrically non-conductive, they should be readily platable using an "electroless plating" operation. This operation is today typically accomplished by conditioning the surface for plating by inorganic etching with a strong oxidizing acid or base, seeding the surface by contact with a noble metal salt solution, e.g., a palladium chloride solution, then immersing the seeded surface in an autocatalytic electroless solution where an initial coating of a conductive metal, e.g., copper or nickel, is established by chemical deposition. The metal coating formed acts as a buss which allows a thicker metal coating to be built up electrolytically.
Adhesion between the metal plate and a polymeric substrate is, however, dependent on the strength of the resin-metal bond.
It has been proposed to improve adhesion by organically pre-etching the surface prior to etching with the inorganic etch. The pre-etch systems have varied widely depending on the nature of the polymer.
Among the organic compounds which have been suggested by the art for the pre-etch treatment of polymeric surfaces have been chlorinated hydrocarbons. For instance, in U.S. Pat. No. 3,445,350 to Klinger et al, several chlorinated hydrocarbons were proposed as "ABS solvents". The chlorinated compounds employed were used in an undiluted state or diluted with a hydrocarbon. The use of chloroform for certain blended polymers is disclosed in U.S. Pat. No. 3,466,232; methylene chloride for polycarbonates in U.S. Pat. No. 3,471,376; emulsified or hydrocarbon diluted halogenated compounds for styrene homopolymers and interpolymers in U.S. Pat. No. 3,533,828; and various emulsions keyed to particular polymers in U.S. Pat. No. 3,570,070.
The use of water insoluble chlorinated compounds for the treatment of polymer surfaces have several inherent deficiencies. Most are high in cost and many hazardous if not toxic. If used without dilution, their continuing evaporation losses is, in itself, an expense. In addition, ventilation equipment uncommon to commercial plating operations have to be installed for personnel safety. Dilution with hydrocarbons does not diminish this problem. Emulsification adds a degree of compatibility with plating operations. Emulsions, however, are difficult to maintain particularly in light of the materials "dragged" into the pre-etch and introduced by errosion of the polymer surface. These materials tend to break emulsions presenting a continuing relatively unpredictable maintenance problem. Emulsion breakdown cannot be readily predicted, and once broken, reconstitution is difficult if not impossible.